In a communication network (e.g., a second generation (2G) Global System for Mobile Communications (GSM) network), system capacity is often limited by co-channel interference (CCI) due to channel re-use.
GSM uses a combination of time division multiple access (TDMA) and frequency division multiple access (FDMA). Gaussian minimum shift keying (GMSK) modulation is used in GSM such that unwanted or spurious emissions outside the nominal bandwidth are sufficiently low to enable adjacent channels to be used from the same base station.
Data transported by a carrier provides up to eight different users by splitting the carrier into eight time slots, using a TDMA scheme. This enables different users of a single radio frequency (RF) channel to be allocated in different time slots. Different users are then able to use the same RF channel without mutual interference. A GSM burst is a transmission made in a time slot, where a burst includes three tail bits at the start of the GSM burst to provide time for a transmitter to ramp up its power, 57 data bits used to carry information, one flag bit to indicate the type of data in the previous field, 26 bits for a training sequence (TS) that is used as a timing reference and for equalization, 1 flag bit to indicate the type of data in the data field, 57 bits of data, 3 tail bits to allow time for a transmitter to ramp down its power, and 8.25 bits to provide a guard time at the end of a GSM burst.
A basic GSM receiver simply treats CCI as additive white Gaussian noise (AWGN), thus resulting in inferior performance. Unlike white Gaussian noise, which is uncorrelated in both time and space, interference typically has a strong correlation in time and space, i.e., the interference is colored. In a wireless communication system with multiple receive antennas, interference may be mitigated through an interference whitening technique, which exploits the correlation of a received signal among multiple receive antennas. Since a GSM system typically uses only a single receive antenna, an interference whitening technique cannot be directly applied. However, a GMSK signal may be approximated by a binary phase shift keying (BPSK) signal with consecutive 90 degree phase rotations. If I and Q are treated as two signal dimensions, BPSK is a one dimensional signal, unlike quadrature phase shift keying (QPSK) and other quadrature amplitude modulation (QAM) schemes that occupy both I and Q dimensions. Therefore, for BPSK or GSMK signals, even with a single receive antenna, it is feasible to perform interference suppression through certain signal space projection, which is commonly referred to as SAIC.